In center pivot irrigation systems, water is directed along a long boom pipe to the field to be irrigated. The boom pivots around a center point on spaced apart towers which support the boom. The towers are generally supported by large pneumatic rubber tires similar to conventional tractor tires. The weight supported by each tower is substantial causing the tires to sink deeply into the ground. As the tires roll across muddy soil, the tires tend to push mud to the sides and front of the tire. The accumulated mud pushed to the front of the tire can eventually build up to the point that the tire can no longer advance. This interrupts operation of the irrigation system until the tire can be pulled out of the rut. The tires on such systems are also subject to extensive wear and tear due to their exposure to the elements, the extensive wet/dry cycles, and the general rough terrain in most fields. Pneumatic tires are thus prone to going flat, which also interrupts operation of the irrigation system until the flat tire is repaired or replaced.
Rotational molding is a method of manufacture for primarily hollow or foamed interior plastic objects. Rotational molding or casting (also known as “roto-molding” and “roto-casting”) is typically used to produce products that are too large or complicated to be produced by other molding techniques, such as thermoforming or injection molding. Some products commonly formed by rotational molding include garbage can, fuel tanks, storage tanks, casks, and toys.
Rotational molding generally utilizes a shell-like, heated hollow mold which is filled with a charge or shot weight of material. The charged mold is then slowly rotated (usually around two, and sometimes three, perpendicular axes) causing the softened material to disperse and stick to the walls of the mold. To maintain even thickness throughout the part, the mold is continuously rotated at all times during the heating and cooling phases. Rotating the mold during cooling minimizes sagging or other deformation of the molded workpiece during the cooling phase. Generally, then, the roto-molding process takes place in four discrete steps: loading or charging the mold, molding or curing under heated conditions, cooling the mold, and lastly unloading the molded part from the now-cool mold.
When the cover to the mold is open, a powdered plastic resin, which is typically colored, is placed into the mold cavity. The cover is then sealed and the mold is placed in a heated environment in which it is rotated. The heat causes the plastic to melt against the heated inside surface of the mold. The mold continues to rotate during heating, and the plastic gradually becomes distributed evenly on the mold walls through gravitational force. While continuing to rotate, the mold is moved out of the oven to a cooling chamber, where the mold and the plastic are cooled to the point that the molded plastic has cured and is sufficient stiff to retain its molded shape. During cooling, the molded resin typically contracts and pulls away from the mold. Once the molded resin is sufficiently cool and rigid, the molded article is removed from the mold. Additional, post-mold machining may be required to yield the final product.